In one known gas monitor, an infra-red source is arranged to emit radiation which passes through a gas to be monitored. Infra-red radiation is absorbed by the gas and that remaining is subsequently detected by a pyroelectric detector. A comparison is made between the source intensity and the intensity of radiation detected following passage through the gas to give the concentration of the gas. The concentration is related to the intensity by the following equation: EQU I=I.sub.o e.sup.-.epsilon.cl
where I is the intensity of radiation detected by the detector, I.sub.o is the intensity of radiation emitted at the source, .epsilon. is effectively a constant which is dependent on the particular gas being monitored, c is the gas concentration and 1 is the distance travelled by the radiation through the gas.
In one known gas monitor, an infra-red source is located remote from a pyroelectric detector on a bench with a tube between them through which gas is passed. Infra-red radiation travels along a direct path between the source and sensor but there also tend to be multiple reflections from the interior surfaces of the tube. This results in numerous different path lengths taken by the infra-red radiation between the source and the sensor, which leads to errors in measuring the gas concentration. Moreover, the errors vary over time because the interior surfaces of the tube gradually degrade and present a non-uniform surface.
The present invention seeks to provide a gas monitor having improved characteristics over those previously known.